1. Field of the Invention
The present invention relates to a method of spraying different spraying materials in depositional direction, a method of manufacturing a sliding member having a sliding surface which shows excellent wear resistance, a piston having a ring groove which shows excellent wear resistance and a method of manufacturing the same.
2. Description of Related Art
In a diesel engine, a top ring groove portion of an aluminum piston is not heat-resistant. Recently, in accordance with a regulation of exhaust gases, it is necessary to control oil consumption, and to make the burning temperature higher. Such needs are more and more severe on a piston ring and a piston, and the above method cannot meet them. Namely, enough oil lubrication or cooling cannot be obtained by a piston ring groove, especially, a top ring groove. So, abrasion occurs between a piston ring and a ring groove.
Conventionally, an attempt that a heat- and wear-resistant layer is formed on a top ring groove portion of a piston by spraying has been made. In spraying, since a base material and a spraying material are freely selected, it is reported that many wear-resistant materials are sprayed. Incidentally, a piston ring groove of a diesel engine of automobiles is a rectangle groove having an inlet of 2 mm and the depth of 5 mm. So, when spraying is performed straight, a spraying angle becomes extremely small, and it is difficult to coat a sprayed layer along the shape of the groove.
On the contrary, as shown in FIG. 17 in Japanese Unexamined Patent Publication (KOKAI) No. 44838/1993, an upper end portion of a groove is chamfered so that a spraying angle can be obtained. In this method, although a sprayed layer is formed on a lower surface of the groove, an upper surface is restored by filling metals. So, a process becomes complicated, and the whole surface of the groove is not treated. Therefore, partial adhesion or abrasion may occur. Furthermore, since the spraying angle is not perpendicular to a treated surface, an adhesion strength of a sprayed coating is declined, and rebound particles are deposited at the groove depth, thereby forming a porous layer. Thus, the quality of the sprayed coating may be deteriorated.
Accordingly, it is an object Of the present invention to provide a piston and a method of manufacturing a sliding member having a sliding surface which comprises a sprayed layer having higher wear resistance and excellent stability as compared with the conventional sprayed layer.
Inventors found out the following matter. When a spraying material comprising not less than two kinds of spraying particles is diagonally sprayed to a surface of an object, spraying particles having low fusion temperature are primarily adhered to the surface of the object, and it is possible to obtain a sprayed layer which contains high ratio of spraying particles having low fusion temperature and whose composition is different from that of the spraying material. They confirmed that the composition of the sprayed layer is somewhat changed by varying the spraying angle to the surface of the object.
Inventors also noticed the following matter. When the sprayed layer is formed by spraying, each spraying particle collides with an object, and it is mashed and shaped like a thin disk, thereby depositing on the surface of the object. Normally, the direction which is perpendicular to depositional direction of the above sprayed layer, namely, the surface on which each spraying particle spreads in the shape of a thin disk is utilized as a sliding surface. They paid an attention to a cut surface which is obtained by cutting the sprayed layer in depositional direction. Then, they thought that the above cut surface shows excellent wear resistance and stable coefficient of friction in consideration of fallout resistance of each spraying particle and the number of each spraying particle which is exposed on the cut surface per unit surface area. The inventors proved the above assumption by experiments.
In a method of spraying a spraying material on a surface of a base material on which a sprayed layer is to be formed, thereby forming a sprayed layer on the surface;
the spraying material comprises not less than two kinds of spraying particles which have each different fusion temperature one another,
a spraying angle which is formed by the surface and spraying direction is set to be small at an initial stage of spraying, and to be large after the initial stage, and
a lower portion of the sprayed layer which is close to the surface contains higher ratio of spraying particles having low fusion temperature and lower ratio of spraying particles having high fusion temperature as compared with an upper portion of the sprayed layer which is far from the surface.
A method of manufacturing a sliding member having a sliding surface which comprises a sprayed layer comprises the steps of:
spraying a granular spraying material on at least a part of a surface of a body portion made of a structural material in at least partially fusing condition,
forming a sprayed layer on the surface, and
forming a sliding surface which comprises a surface obtained by grinding or cutting the sprayed layer.
The spraying material is sprayed in the direction which is parallel to or diagonal to the sliding surface, so the spraying material is deposited in the direction which is perpendicular to the sliding surface. The sliding surface is a section of the deposited spraying material which is obtained by grinding or cutting the deposited spraying material in depositional direction.
A piston having at least one ring groove at an outer periphery which is slided and brought into contact with an inner periphery of a cylinder comprises:
a piston body having a broad groove which is broader than the ring groove at the outer periphery,
a sprayed layer which is formed by spraying a spraying material into the broad groove of the piston body in the direction which is perpendicular to the outer periphery, and depositing the spraying material in the broad groove in the direction toward depth, and
a ring groove which is formed by grinding or cutting the sprayed layer in depositional direction.
A method of manufacturing a piston having at least one ring groove at an outer periphery which is slided and brought into contact with an inner periphery of a cylinder comprises the steps of:
forming a broad groove which is broader than the ring groove at the outer periphery,
primarily spraying a spraying material, which comprises not less than two kinds of spraying particles having each different fusion temperature one another, in the direction which is diagonal to the surface of the broad groove at low spraying angle,
forming a lower sprayed layer which contains higher ratio of spraying particles having low fusion temperature,
secondarily spraying the spraying material on the lower sprayed layer at higher spraying angle than that of the lower sprayed layer,
forming an upper sprayed layer which contains lower ratio of spraying particles having low fusion temperature, and
forming a ring groove in the upper sprayed layer.
In the present invention, the spraying material comprises not less than two kinds of spraying particles having each different fusion temperature one another, and the composition ratio of the sprayed layer can be changed by varying the spraying angle. As a result, it is possible that the lower portion of the sprayed layer contains higher ratio of material having high affinity to the base material, and that the upper portion of the sprayed layer contains higher ratio of material having some characteristics which is desirable for the sprayed layer.
In the method of manufacturing the sliding member having the sliding surface which comprises the sprayed layer according to the present invention, the sprayed layer is formed by depositing the spraying material on the surface of the sliding member on which the sprayed layer is to be formed. After that, the sliding surface is formed in the depositional direction of the sprayed layer, and the sliding member can be obtained. The edge surface of each spraying material which is disposed in the shape of a thin disk by spraying is exposed on the sliding surface. As a result, the area of each spraying material which is exposed on the sliding surface is narrow. The sliding surface is formed by a large number of spraying materials. So, the sliding surface hardly shows friction characteristics of a specific spraying material or some spraying materials. It exhibits average friction characteristics of whole spraying materials. Therefore, stable coefficient of friction can be obtained.
Each spraying material is arranged in such a manner that it stands against the sliding surface. One end of each sprayed layer forms the sliding surface, and the other end of each sprayed layer is inside and far from the sliding surface. Each spraying material for forming the sliding surface is hardly comes off from the sliding surface. So, abrasion which is caused by fallout hardly occurs. Since the area of one piece of the spraying material which is exposed on the sliding surface is narrow, stress which is acted on one piece of spraying material is small. Therefore, fallout of the spraying material comes to rarely occur, and wear resistance becomes excellent.
Such characteristics is suitable for the piston of the present invention, and the ring groove of the piston shows remarkably excellent wear resistance.
In the method of manufacturing the piston of the present invention, the afore-mentioned method of spraying material is used for the method of manufacturing the piston, and it is possible to form the upper sprayed layer and the lower sprayed layer by using the same spraying material. Furthermore, since the ring groove is formed in the upper sprayed layer, the ring groove can obtain high wear resistance and can be operated with the piston body integrally.